The present invention relates to end fittings for water-cooled stator bars used in electrical generators (typically known as “stator bar clips”) in which a corrosion protective coating is applied to the interior brazed joints of the end fittings. The invention also relates to a new method for applying prescribed amounts of the protective coating to the stator bar end fittings of both new and in-service generators. The product and method according to the invention improve the lifetime of stator bar clips and significantly reduce the possibility of leaks over time through the stator bar fluid channels and connections.
The stator windings in electrical generators typically consist of a plurality of insulated copper bars assembled in slots and brazed into copper clips on the ends to form bar assemblies. Each stator bar includes a combination of small rectangular solid and hollow copper conductors known as “strands,” that are brazed to one another within the stator bar and brazed to the interior walls of an end fitting. The hollow strands provide the means for circulating coolant, typically de-ionized water, through the strands to prevent the generator from overheating by removing heat from the insulated stator bars. An end fitting typically includes an enclosed chamber for ingress or egress of the stator bar cooling liquid. That is, the fitting provides a hollow chamber that serves to manifold the water passages of the strands. Another opening of the end fitting receives the ends of the strands of the stator bar, typically with the outermost copper strands of the stator bar brazed to one another.
During normal operation of the generator, leaks can develop over time at or near the connection between the ends of the stator bars and the end fitting, as well as between adjacent copper strands. The end fitting/copper strand connection, as well as the strand-to-strand joints have the highest potential for causing damage to the generator if a leak occurs because a leak at those locations could flow directly into the ground wall insulation of the generator winding. The likely result of such a leak would be a degradation of the dielectric strength of the insulation and possible failure at or near the winding. Based on past experience, such leaks generally result from a corrosion process that begins on the interior surface of the brazed joint of the water-cooled stator bar clips. The leak mechanism is believed to result from a two-part corrosion process initiated at the surface of the brazed joint where stagnant water tends to reside in small cavities created during the brazing of concave joint surfaces between adjacent strands.
In the past, copper and its allows have been used for stator bars because of their generally good corrosion resistance, moderate costs and advantageous dielectric properties and thermal conductivities. Copper is considered a noble metal and will react in environments with oxygen present. Thus, copper is vulnerable to eventual corrosion and failure over time. Localized corrosion of water-cooled stator bars normally occurs in areas where the oxygen-free high conductivity (“OFHC”) copper contacts the brazing alloy. More severe corrosion can also occur in creviced regions between the OFHC copper and the brazing alloy, or in small voids formed during the brazing process. The crevices tend to limit the flow of cooling liquid into and around localized areas of the metal surface.
The occurrence of crevice corrosion thus depends to some degree on the specific crevice geometry and the alloy combination involved. Localized environmental changes in a “stagnant” area can also result in the formation of acidic conditions (typically phosphoric acid) that result in the initiation and propagation of unwanted crevice corrosion.
The field repair of leaks occurring through stator bar end connections has only been moderately successful in the past. Frequently, the inability to repair leakage through the stator bar end connections forces generator owners to replace the leaking bars or the entire stator winding in order to eliminate the leaks. Such in-kind replacements with entirely new components can be expensive and require significant generator downtime.
Many concerns over stator bar leaks relate to the brazing process itself and hence the hydraulic integrity of the resulting brazed joint. Past experience with the brazing process shows that completely sound repair brazes are difficult to achieve because of the relatively large surface area involved, as well as the inability to effectively feed a braze alloy into the joint during solidification. Consequently, some unwanted porosity can occur in the brazed joints themselves or at the joint surface, resulting in rework or scrapping of the stator bars. Worse, the repairs can create conditions that actually enhance subsequent corrosion and/or erosion after the generator is placed back into service.
One known method of repairing stator bars is described in commonly-owned U.S. Pat. No. 5,581,869, entitled “Repair Method for Sealing Liquid-Cooled Stator Bar End Fittings For a generator.” The '869 patent describes and claims a repair technique that involves the on-site removal of a majority of the original end fitting such that only a continuous ring surrounding the strand bundle remains. The exterior portion of the ring is machined to an acceptable tolerance for brazing with a replacement end fitting. The repair alloy used in the repair process, like the original alloy, typically consists of a copper phosphorous alloy but with a higher concentration of silver or other element to ensure that the repair alloy will have a lower melting point temperature than the original alloy. The machined ring that surrounds the exterior periphery of the strands is then inserted into a replacement end fitting and brazed without disturbing the prior braze.
Another known repair method coats the surfaces of strands in the stator bar clips with epoxy to protect them from erosion. Other known methods for repairing leaking stator bars involve varying degrees of generator disassembly to fix individual stator bar clips, in some cases requiring complete removal and replacement of the stator bars. Again, considerable generator downtime is necessary to gain access to the stator bars, making such repairs costly and time consuming.
Typical of other known prior art repair methods is U.S. Pat. No. 5,557,837 to Thiard-Laforet et al which discloses a method for repairing stator winding bars whereby narrow slots are introduced between adjacent conductor elements into the end of the bar in the transverse and vertical directions. A first connecting part surrounding the bar is pushed on, and the outer surfaces of the bar end are filled with copper foil and/or solder foil and then inductively heated and soldered. Again, such repair methods are costly and time-consuming and do not always solve an inherent corrosion problem, namely the increased likelihood of localized corrosion at or near stagnant water zones.